Air filter

ABSTRACT

There is provided an air filter, which has a long lifetime and permits to capture effectively carbon particles and reduce costs. The air filter has the first filter layer  11  impregnated with oil and the second filter layer  12 , which is provided on the downstream side of the first filter layer  11  and serves as a lipophobic layer having an oil-repellent property. The filter material of the first filter layer  11  has a higher density than the filter material of the second filter material  12.

TECHNICAL FIELD

The present invention relates to an air filter, and especially to an airfilter, which permits to capture efficiently carbon particles.

BACKGROUND OF THE INVENTION

As an air filter for a vehicle, there has been known a wet-type filterhaving filter paper impregnated with oil. Viscose oil is generally usedas impregnation oil. Accordingly, it is called the “viscose type” airfilter. The viscose type air filter has an advantage of extendedlifetime in comparison with the dry-type filter, which is notimpregnated with oil. In the viscose type air filter, oil on the surfaceof the filter captures dust and then the oil penetrates into thecaptured dust so as to provide a function of capturing the other dust onthe surface of the captured dust. Such a chain capture of dust permitsto increase an amount of captured dust per volume of the filter.

Japanese Utility Model Publication No. S63-27767 describes the improvedviscose type air filter. The air filter described in Japanese UtilityModel Publication No. S63-27767 is a laminate of the first filtermaterial having a relatively high density and the second filter materialhaving a relatively low density. Such a laminate structure permits tomake oil impregnation ratios in the first and second filter materialsdifferent from each other, resulting in prevention of clogging of thefilter, providing an effective capture of dust.

The wet-type filter impregnated with oil has a problem that carbonparticles cannot be captured effectively. The dry-type filter, which isnot impregnated with oil, can capture effectively carbon, but has aproblem of a decreased amount of captured dust per volume of the filter.Use of non-woven fabric leads may cause problems that dust permeationoccurs at a high flow rate and in a serious pulsation and costsincrease.

DISCLOSURE OF THE INVENTION

An object of the present invention, which was made in view of theabove-described circumstances, is therefore to provide an air filter,which permits to provide a large amount of captured dust per volume ofthe filter, capture effectively carbon particles and reduce costs.

The present invention will be described below. Reference numerals in theaccompanying drawings will be given with parentheses in order tofacilitate understanding of the present invention. However, the presentinvention is not limited only to embodiments as illustrated.

In order to attain the aforementioned object, an air filter (10)comprises: a first filter layer (11) impregnated with oil; and a secondfilter layer (12) provided on a downstream side of said first filterlayer, said second filter layer being composed of a lipophobic layerhaving an oil-repellent property, each of said first filter layer (11)and said second filter layer (12) being formed of filter paper, filtermaterial of said first filter layer (11) having a higher density thanfilter material of said second filter layer (12), and said second filterlayer (12) being composed of said lipophobic layer over an entirethickness.

According to the present invention, the first filter layer that isformed of the filter paper having the high density and is impregnatedwith oil, captures dust on the one hand, and the second filter layerthat is formed of the filter paper having the low density and subjectedto an oil-repellent treatment so as not to be impregnated with oil,captures carbon particles, on the other hand, thus performing effectivefiltering operations by means of both filter layers. In general, thethickness of the second filter layer is determined on the basis ofspecification of capturing the carbon particles. According to thepresent invention, the second filter layer has the oil-repellentproperty over the entire thickness. Consequently, the oil in the firstfilter layer is not carried by any portion of the second filter layer inits thickness direction, thus making it possible to cause the secondfilter layer over its entire thickness to capture effectively the carbonparticles. On the contrary, in case where the second filter layer isprovided only in the upstream portion with a lipophobic layer, the oilin the first filter layer may penetrate through the lipophobic layer,thus causing a problem that the carbon particles cannot be capturedeffectively over the entire thickness of the second filter layer. Eachof the filter layers is formed of filter paper and it is thereforepossible to prevent occurrence of dust permeation at a high flow rateand in a serious pulsation and reduce material cost to the minimum. Inaddition, the filter material of the first filter material has a higherdensity than the filter material of the second filter layer and pressureloss can also therefore be minimized.

An embodiment of the present invention is characterized in that saidfirst filter layer and said second filter layer are combined integrallywith each other.

According to the present invention, the first filter layer and thesecond filter layer can be combined integrally with each other, thusproviding a simple layer structure. The entire thickness of the filtercan be adjusted, as an occasion demands.

An embodiment of the present invention is characterized by furthercomprising an additional layer.

According to the present invention, the additional layer as included canimprove performance of the filter element.

An embodiment of the present invention is characterized in that saidfirst filter layer has a pore size of from 70 μm to 120 μm and saidsecond filter layer has a pore size of from 100 μm to 180 μm.

Grounds for limiting the pore size of the first filter layer of from 70μm to 120 μm and the pore size of the second filter layer of from 100 μmto 180 μm are described below. First, description will be given ofgrounds for limiting the pore size of the first filter layer of from 70μm to 120 μm. The present inventors made an experiment on an amount ofexperimental dust established by the JIS (Japanese Industrial Standard)#8, which penetrates through the first filter, while gradually changingthe pore size. FIG. 8 shows the results of the experiment. An abscissadenotes the pore size (μm) and an ordinate shows an amount of captureddust according to the JIS #8, which penetrates the first filter. It isrecognized from FIG. 8 that, with the pore size of over 120 μm, a sievediameter becomes large so as to increase an amount of the dust, whichpenetrates the first filter, deteriorating purification efficiency. Onthe contrary, with the pore size of less than 70 μm, an amount of thedust, which penetrates the first filter, is rapidly decreased.Consequently, an amount of the dust captured by the first filter rapidlyincreases, thus decreasing the lifetime of the filter.

Then, description will be given of grounds for limiting the pore size ofthe second filter layer of from 100 μm to 180 μm. The present inventorsmade an experiment on an amount of carbon, which penetrates through thesecond filter, while gradually changing the pore size. FIG. 9 shows theresults of the experiment. An abscissa denotes the pore size (μm) and anordinate shows an amount of captured carbon, which penetrates the secondfilter. It is recognized from FIG. 9 that, with the pore size of over180 μm, a sieve diameter becomes large so as to increase an amount ofthe carbon, which penetrates the second filter, deterioratingpurification efficiency. On the contrary, with the pore size of lessthan 100 μm, an amount of the carbon, which penetrates the secondfilter, is rapidly decreased. Consequently, an amount of the dustcaptured by the first filter rapidly increases, thus decreasing thelifetime of the filter.

Limiting the pore sizes of the first filter layer and the second filterlayer within the above-mentioned ranges makes it possible to increasethe lifetime of the filter, without deteriorating purificationefficiency of the filter.

An embodiment of the present invention is characterized in that saidsecond filter layer has a downstream end, which is exposed.

According to the present invention, it is possible to prevent oil fromadhering to the downstream end of the second filter, permitting a moreeffective capture of the carbon particles utilizing the second filterlayer.

The air filter may be manufactured by subjecting the second filter layerto an oil-repellent treatment and then combining the first filter layerand the second filter layer integrally with each other, or by combiningthe first filter layer and the second filter layer integrally with eachother, subjecting the second filter layer to the oil-repellent treatmentand then impregnating the first filter layer with oil.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating an embodiment of an airfilter of the present invention;

FIG. 2 is a cross-sectional view illustrating a filter element of thefirst embodiment;

FIG. 3 is a cross-sectional view illustrating the filter element of thesecond embodiment (FIG. 3(A) shows the filter element as beingmanufactured and FIG. 3(B) shows the filter element as completelymanufactured);

FIG. 4 is a cross-sectional view illustrating the filter element of thethird embodiment;

FIG. 5 is a view illustrating generally the filter element of the fourthembodiment;

FIG. 6 is a view illustrating generally the filter element of the fifthembodiment;

FIG. 7 is a view illustrating a device for measuring a pore size;

FIG. 8 is a graph illustrating the relationship between the pore sizeand an amount of dust captured in the first filter layer; and

FIG. 9 is a graph illustrating the relationship between the pore sizeand an amount of carbon captured in the second filter layer.

BEST MODE FOR CARRYING OUT THE INVENTION

Now, embodiments of an air filter of the present invention will bedescribed below with reference to FIGS. 1 to 6.

An air filter 10 as shown in FIG. 1 is composed of a pleated filterelement 1 and a frame member 2, which is formed of plastic into arectangular shape to support the filter element 1. As shown in FIG. 1,the filter element 1 is subjected to an insert injection to form theframe member 2 to which a peripheral portion of the filter element 1 isfixed.

The filter element 1 a of the first embodiment as shown in FIG. 2 isprovide with the first filter layer 11 having filter element impregnatedwith oil such as viscose oil and the second filter layer 12, which iscomposed of a lipophobic layer having an oil-repellent property. Boththe first filter layer 11 and the second filter layer 12 are formed offilter paper. The filter material of the first filter layer 11 has ahigher density than the filter material of the second filter layer 12.The second filter layer 12 is formed as the lipophobic layer over theentire thickness, i.e., from the upstream-side end face 12 a to thedownstream-side end face 12 b so as to prevent or inhibit oilimpregnated in the first filter layer 11 from entering the second filterlayer 12 under the function of the oil-repellent property of thelipophobic layer. The downstream-side end face 12 b of the second filterlayer 12 is exposed to come into contact with air. The upper surface ofthe first filter layer 11 oozes with the oil impregnated in the firstfilter layer 11 to provide a condition in which dust can easily becaptured. Imparting the oil-repellent property to the second filterlayer 12 makes it possible to provide a filter layer for capturingcarbon particles, which have not been captured by the first filter layer11 impregnated with the oil and passed through the first filter layer11, under the function of contact filtration.

The lipophobic property can be imparted to the second filter layer 12with the use of material as properly selected, having the oil-repellentproperty such as filter paper impregnated with fluorine contained resin.The first filter layer to be impregnated with oil by the subsequent stepand the second filter layer, which is formed of filter paper to beimpregnated with the above-mentioned fluorine contained resin may becombined together in the paper manufacturing process. It is alsopossible to provide a combined body of the above-mentioned two layersutilizing a method comprising the steps of combining the first filterlayer and the second filter layer together (for example into a filterlayer having a single layer structure), impregnating the first filterlayer (for example the upper half portion of the above-mentioned singlefilter layer) with oil and applying lipophobic agent to the secondfilter layer (for example the lower half portion of the above-mentionedsingle filter layer). In such a case, an application method such as aspraying method, a roller coating method (such as a direct rollercoating method and a kiss-roll coating method), a dipping method and theother method is applicable. Such an application method providesadvantages of forming the lipophobic layer with an easy process.

In the second embodiment as shown in FIG. 3(A), the filter element 1 bis obtained by combining the first filter layer 11 having the filtermaterial impregnated with oil and the second filter layer 12 composed ofthe lipophobic layer by an adhesive layer 13. The first filter layer 11and the second filter layer 12 are joined together so as to permit airto pass through the contacting surfaces of these layers. Aftercompletion of the manufacturing steps, the adhesive agent 13 permeatesinto the first filter layer 11 and the second filter layer 12 so as notto form any gap between the first filter layer 11 and the second filterlayer 12 as shown in FIG. 3(B). Also in the embodiment the upper surfaceof the first filter layer 11 oozes with the oil impregnated in the firstfilter layer 11 to provide a condition in which dust can easily becaptured. Imparting the oil-repellent property to the second filterlayer 12 makes it possible to provide a filter layer for capturingcarbon particles under the function of contact filtration. Theembodiment describes the state in which no gap is formed between thefirst filter layer 11 and the second filter layer 12 after completion ofthe manufacturing steps. The adhesive layer 13 may however be leftbetween the first filter layer 11 and the second filter layer 12 so thatthese layers 11 and 12 are apart from each other.

In the method for manufacturing the filter element of the embodiment,the first filter element 11 and the second filter element 12 are placeone upon another through the adhesive agent to combine them together andthen the first filter layer 11 is impregnated with oil.

As the adhesive agent used for the adhesive layer 13, it is preferableto select and use for example a hot-melt material, taking intoconsideration the manufacturing steps of the filter and conditions underwhich the filter as manufactured is used. Such a hot-melt material mayinclude olefin material or polyester material.

In the first and second embodiments, an appropriate value may beselected, as the pore size of the first filter layer 11, for examplefrom the range of 70 μm to 120 μm in accordance with a performancerequired for the air filter. The capturing efficiency of dust in thefirst filter layer 11 is determined so that the filter sieve can capturethe dust. Consequently, the value of the pore size is set taking intoconsideration the particle size of the dust to be captured and pressureloss.

The pore size of the second filter layer 12 having the lipophobicproperty is set for example to a value within the range of from 100 μmto 180 μm. Such a setting leads to the pore size of the entire filterelement of 70 μm to 120 μm. A reason for using the filter materialhaving a relatively large pore size for the second filter layer 12 isthat the second filter layer 12 is to be used to capture carbonparticles under the function of contact filtration and it is unnecessaryto make the filter material dense. Making the filter material of thesecond filter layer 12 sparser than the filter material of the firstfilter layer 11 can rather reduce pressure loss.

The filter element 1 c of the third embodiment as shown in FIG. 4 isprovided with the first filter layer 11 having the filter materialimpregnated with oil, the first intermediate layer 15, the secondintermediate layer 16 and a clean layer 17, which are disposed in thisorder from the upstream side to the down stream side. In the embodiment,the second filter layer composed of the lipophobic layer having theoil-repellent property serves as any one of the first intermediate layer15, the second intermediate layer 16 and the clean layer 17. Withrespect to the other layer, material, a pore size, thickness and theother conditions may be determined taking into consideration object ofuse of the filter. The material for the other layer may be formed forexample of filter paper or non-woven fabric. In case where the filterpaper is used, a water-repellent treatment applied to it makes itpossible to prevent water from being sucked into the inside of anengine, even when a suction port of an intake system is located in aplace in which water is easily be sucked.

Now, a filtering operation of the air filter of the embodiment of thepresent invention will be described. An arrow with a reference symbol“A” in FIGS. 1 to 4 denotes a flowing direction of air. As shown inFIGS. 2 to 4, air first passes through the first filter layer 11 andthen through the second filter layer.

As described above, the first filter layer 11 is a wet-type filterlayer, which is impregnated with oil. A dense structure of the filtermaterial and existence of oil impregnated therein are essential toeffective capture of dust. A condition that the filter layer, which isdense and impregnated with oil, exists so as to face an air passage, isalso essential to a sufficient capture of dust. Accordingly, use of thefirst filter layer 11, which satisfies these conditions, makes itpossible to effectively capture dust in the air. However, the filtermaterial impregnated with oil cannot generally provide an effectivecapturing effect of carbon particles in the air. More specifically, whatcan effectively be captured by means of the first filter layer 11, isdust other than the carbon particles.

The air, which has passed through the first filter layer 11, enters thesecond filter layer 12. The second filter layer 12, which is thelipophobic filter having the oil-repellent property and has the sparse(i.e., low density) filter material, can provide an effective capture ofthe carbon particles. The second filter layer 12 may capture the carbonparticles under the function of contact filtration.

According to the air filter of each embodiment of the present invention,the first filter layer 11 effectively captures dust other than carbonparticles and the second filter layer 12 effectively captures, over itsentirety in the thickness direction, the carbon particles, which havepassed through the first filter layer 11. Consequently, the air filteras a whole effectively captures both of the carbon particles and theother dust. In the embodiment of the present invention, the secondfilter layer 12 captures the carbon particles under the function ofcontact filtration, unlike the conventional air filter in which thefilter materials are disposed so that the density of the filtermaterials becomes higher in the downstream direction of the flow of air.As a result, it is possible to make the density of the filter materialof the second filter layer 12 lower than that of the first filter layer11, thus reducing pressure loss.

In the wet-type filter, oil generally moves to the downstream side offlow of air through a phenomenon called “carrying off” by air. In theair filter of the embodiment of the present invention, the second filterlayer 12 serving as the lipophobic layer is provided on the downstreamside of the first filter layer 11. There is ensured a state in which oildoes not easily moves to the second filter layer 12 under the functionof the oil-repellent property of the lipophobic layer.

In general, an amount of oil impregnated in the filter layer has aninfluence on the property of capturing carbon particles so thatimpregnation of the filter layer with oil lower the capturing property.Accordingly, if the second filter layer 12 is impregnated with oil, thecarbon particles cannot be captured effectively. In the embodiment ofthe present invention, the second filter layer 12 is the lipophobiclayer as described above so as to prevent the oil from moving. As aresult, almost no movement of oil from the first filter layer 11 to thesecond filter layer 12 occurs. The second filter layer can be kept freefrom oil, thus making it possible to maintain a high capturing propertyby which carbon particles can be captured effectively for a long periodof time.

In addition, oil does not easily move to the second filter layer 12 andthere is no possibility that the oil is carried to the furtherdownstream side of the second filter layer 12. Accordingly, when the airfilter of the embodiment of the present invention is mounted on anintake system of an engine for a vehicle, there occurs no inconveniencethat the oil is sucked into the downstream side of the air filter. Incase where the third filter layer and the fourth filter layer areprovided on the downstream side of the second filter layer 12, it istherefore preferable to arrange them so that the densities of thesefilter layers becomes lower in the flowing direction of air.

In case where the air filters in both embodiments of the presentinvention are used as an air filter for a vehicle, it is preferable tolimit the entire thickness of the filter element 1 within the range offrom 0.80 mm to 3.50 mm, and more preferably within the range of from0.90 mm to 1.75 mm. In such a case, it is preferable to limit thethickness of the first filter layer within the range of from 0.3 mm to0.75 mm and the second filter layer within the range of from 0.6 mm to1.00 mm.

In the embodiment of the present invention, the filter element 1 ispleated so as to increase the substantial area of the filter, as shownin FIGS. 1 to 6. The filter element 1 may be used in an extended flatstate as shown in FIG. 5(c). FIGS. 5(a) and 5(b) illustrate examples ofthe filter elements 1, which are pleated into a panel-shaped filter. Itis also possible to form the filter elements 1 into a tubular shape anda chrysanthemum shape as shown in FIGS. 6(a) and (b), respectively, sothat air can pass through the filter elements 1 from the inside to theoutside, and vice versa. The shape of the filter element can be selectedfreely in such a manner.

Brief description will be given below of a testing method for measuringthe pore size set forth in the above-described embodiments, withreference to FIG. 7. A disk-shaped filter 23, which serves as an sampleand having a diameter of 29 mm, is held air-tightly between two aluminumpipes 21, 22. The lower aluminum pipe 22, which is placed below thefilter 23, is provided on the lower end side with a valve 26 formaintaining a constant pressure of air in the pipe 22. The valve 26 isconnected to an air supply device 28 having an air compressor.

An air sampler pipe 25 is fitted to the side surface of the aluminumpipe 22. The air sampler pipe 25 is connected to a pressure gauge 27.The pressure gauge 27 can measure the pressure of the inside 24 of thealuminum pipe 22 in this manner.

Prior to measurement of the pore size, the aluminum pipe 21, which isplaced above the filter 23, is filled with isopropyl alcohol. The inside24 of the aluminum pipe 22 is equal to the atmospheric pressure. Then,the valve 26 is gradually opened to supply air from the air supplydevice 28 so as to gradually increase the pressure of the inside 24 ofthe aluminum pipe 22. Pressure difference between the atmosphericpressure and the inside 24 of the aluminum pipe 22 is measured utilizingthe pressure gauge 27 at the time when a single bubble first comes fromthe upper surface of the filter 23 into the isopropyl alcohol.

A pore size (μm) is obtained on the basis of the thus obtained pressuredifference utilizing a conversion table indicated below. The conversiontable as conventionally used utilizes a unit “mmaq” for the pressure. Aunit “Pa” according to the SI unit system is also indicated in thefollowing table.

TABLE 1 PRESSURE PRESSURE PORE SIZE PRESSURE PRESSURE PORE SIZE PRESSUREPRESSURE PORE SIZE mmAq Pa μm mmAq Pa μm mmAq Pa μm 50 490.0 254.3 84823.2 128.6 118 1156.4 86.0 51 499.8 247.2 85 833.0 126.7 119 1166.285.2 52 509.6 240.5 86 842.8 124.9 120 1176.0 84.4 53 519.4 234.1 87852.6 123.2 121 1185.8 83.6 54 529.2 228.1 88 862.4 121.5 122 1195.682.8 55 539.0 222.3 89 872.2 119.8 123 1205.4 82.0 56 548.8 216.9 90882.0 118.2 124 1215.2 81.3 57 558.6 211.7 91 891.8 116.7 125 1225.080.5 58 568.4 206.7 92 901.6 115.2 126 1234.8 79.8 59 578.2 202.0 93911.4 113.7 127 1244.6 79.1 60 588.0 197.5 94 921.2 112.2 128 1254.478.4 61 597.8 193.2 95 931.0 110.8 129 1264.2 77.7 62 607.6 189.0 96940.8 109.5 130 1274.0 77.0 63 617.4 185.1 97 950.6 108.1 131 1283.876.4 64 627.2 181.3 98 960.4 106.8 132 1293.6 75.7 65 637.0 177.6 99970.2 105.5 133 1303.4 75.1 66 646.8 174.1 100 980.0 104.3 134 1313.274.4 67 656.6 170.8 101 989.8 103.1 135 1323.0 73.8 68 666.4 167.5 102999.6 101.9 136 1332.8 73.2 69 676.2 164.4 103 1009.4 100.7 137 1342.672.6 70 686.0 161.4 104 1019.2 99.0 138 1352.4 72.0 71 695.8 158.5 1051029.0 98.5 139 1362.2 71.4 72 705.6 155.7 106 1038.8 97.4 140 1372.070.8 73 715.4 153.0 107 1048.6 96.3 141 1381.8 70.3 74 725.2 150.4 1081058.4 95.3 142 1391.6 69.7 75 735.0 147.9 109 1068.2 94.3 143 1401.469.2 76 744.8 145.5 110 1078.0 93.3 144 1411.2 68.5 77 754.6 143.1 1111087.8 92.3 145 1421.0 68.1 78 764.4 140.8 112 1097.6 91.4 146 1430.867.6 79 774.2 138.6 113 1107.4 90.4 147 1440.6 67.1 80 784.0 136.5 1141117.2 89.5 148 1450.4 66.6 81 793.8 134.4 115 1127.0 88.6 149 1460.266.1 82 803.6 132.4 116 1136.8 87.7 150 1470.0 65.8 83 813.4 130.4 1171146.6 86.9 151 1479.8 65.1

According to the present invention, the first filter layer, which isformed of a dense filter paper and impregnated with oil, captures dustand the second filter layer, which is formed of a sparse filter paperand is not impregnated with oil, captures carbon particles so that theboth filter layers can perform an effective filtration. In addition, thesecond filter layer has the lipophobic property over its entirethickness. The oil of the first filter layer is not carried to anyportion of the second filter layer, thus making it possible for thesecond filter layer to capture effectively the carbon particles over itsentire thickness. Further, the filter material of the second filterlayer is sparser than the filter material of first filter layer, thusreducing pressure loss.

1. An air filter comprising: a first filter layer formed of a firstfilter paper material having a predetermined density, said first filterlayer being impregnated with oil; and a second filter layer provided ona downstream side of said first filter layer so as to be independentfrom said first filter layer, said second filter layer being formed of asecond filter paper material having a lower density than saidpredetermined density of said first filter layer, said second filterlayer being impregnated over its entirety with an oil-repellent agent.2. The air filter as claimed in claim 1, wherein: said first filterlayer and said second filter layer are combined integrally with eachother.
 3. The air filter as claimed in claim 1, further comprising anadditional layer.
 4. The air filter as claimed in claim 1, wherein: saidfirst filter layer has a pore size of from 70 μm to 120 μm and saidsecond filter layer has a pore size of from 100 μm to 180 μm.
 5. The airfilter as claimed in claim 1, wherein: said second filter layer has adownstream end, which is exposed.
 6. The air filter as claimed in claim2, wherein: said second filter layer is subjected to an oil-repellenttreatment and then said first filter layer and said second filter layerare combined integrally with each other.
 7. The air filter as claimed inclaim 2, wherein: said first filter layer and second filter layer arecombined integrally with each other; and then, said second filter layeris subjected to an oil-repellent treatment and said first filter layeris impregnated with oil.
 8. The air filter as claimed in claim 2,further comprising an additional layer.
 9. The air filter as claimed inclaim 3, wherein: said first filter layer has a pore size of from 70 μmto 120 μm and said second filter layer has a pore size of from 100 μm to180 μm.
 10. The air filter as claimed in claim 3, wherein: said secondfilter layer is subjected to an oil-repellent treatment and then saidfirst filter layer and said second filter layer are combined integrallywith each other.
 11. The air filter as claimed in claim 3, wherein: saidfirst filter layer and second filter layer are combined integrally witheach other; and then, said second filter layer is subjected to anoil-repellent treatment and said first filter layer is impregnated withoil.
 12. A air filter, comprising: a first layer of a first filter paperimpregnated with oil; and a second layer of a second filter paper placedin a downstream air direction adjacent the first layer, a density of thefirst filter paper being greater than a density of the second filterpaper, the second layer formed as an oil-repellent lipophobic layer overan entire thickness of the second layer.
 13. The filter of claim 12,wherein, a downstream face of the second layer is exposed to air, anupper, upstream face of the first layer oozes with the oil.
 14. Thefilter of claim 12, wherein the second layer is impregnated with a resincontaining fluorine.
 15. The filter of claim 12, further comprising: anadhesive layer binding the first layer with the second layer with thefirst and second layers contacting one another, the adhesive layerpenetrating a downstream side of the first layer and an upstream side ofthe second layer.
 16. The filter of claim 15, wherein the adhesive layercomprises one of an olefin material and a polyester material.
 17. Thefilter of claim 12, wherein, the first layer has a pore size of from 70μm to 120 μm and the second filter layer has a pore size of from 100 μmto 180 μm.
 18. A air filter, comprising: a first layer of a first filterpaper impregnated with oil; a second layer of a second filter paperplaced in a downstream air direction contacting the first layer; and ahot-melt adhesive layer binding the first layer with the second layer,the adhesive layer penetrating a downstream side of the first layer andan upstream side of the second layer, a density of the first filterpaper being greater than a density of the second filter paper, thesecond layer formed as an oil-repellent lipophobic layer over an entirethickness of the second layer.
 19. The filter of claim 18, wherein thesecond layer is impregnated with a resin containing fluorine.
 20. Thefilter of claim 18, wherein, the first layer has a pore size of from 70μm to 120 μm and the second filter layer has a pore size of from 100 μmto 180 μm.